1. Field of the Invention
This invention relates to digital mobile communications terminal equipment and a receiving method therefor.
2. Description of Related Art
In mobile communications, it is possible to assume that its transmission line is a multiplex wave propagation path which includes a plurality of arrival routes of radio waves for a mobile station due to the influence of configuration of the land and buildings around the mobile station. In high speed transmission, wherein the difference in time between the arrival of advanced waves and delayed waves in this multiplex wave propagation is equivalent to a length of a symbol, frequency selective fading takes place, which causes an intersymbol interference between the advanced and delayed waves. An equalizer of each terminal equipment is provided for detecting a transmission signal from received data which suffer from such intersymbol interference.
An exemplary conventional receiving circuits including such an equalizer is shown in FIG. 1. Referring to FIG. 1, a signal received through antenna 25 successively passes high frequency amplifier 26, intermediate frequency amplifier 27, demodulator 28 and equalizer 31 to be demodulated. Equalizer 31 includes transmission path estimating section 29 and equalizing section 30, and the output of demodulator 28 is connected to both propagation path estimating section 29 and equalizing section 30.
General operation of equalizer 31 is described subsequently. Propagating path estimating section 29 detects an auto-correlation of a known preamble bit train contained in received signal data inputted from demodulator 28 and effects estimation of a propagating path using a bit train wherein this auto-correlation function makes an impulse. A result of estimation obtained by the propagating path estimation, that is, a parameter of the amount of the intersymbol interferences, is sent to equalizing section 30, at which reverse conversion using the intersymbol interferences is preformed to equalize the signal data into data which would have been transmitted from a transmitter.
The conventional receiving circuit including an equalizer is disadvantageous in that, if the communication quality is deteriorated by frequency selective fading or the like, then the received signal level drops and the error rate of demodulated data becomes high. As a countermeasure, a diversity receiving system based on comparison in received signal level is employed. However, such a diversity receiving system has a drawback in that, since the received signal level and the intersymbol interference amount have no correlation between them, even if a diversity branch of a high received signal level is selected, deterioration in characteristic caused by intersymbol interference cannot be improved. Further, diversity signal reception requires two or more receiving circuits, and consequently, there is the drawback that reduction in power consumption cannot be achieved. It is another drawback that, even if the received signal level drops to such a degree that data obtained by demodulation are in error at an error rate higher than an anticipated level and accordingly cannot be adopted as demodulated data, equalizing processing is still performed for such data, and consequently, unnecessary circuit operation is performed and excessive power consumption is wasted.